Testing the Space-Bound Submarines That Will Explore Alien Oceans

In the coming decades, we could send a manned mission to an asteroid, put a person on Mars, or construct a space station and telescope on the moon. But the most revolutionary discoveries could come from autonomous robots exploring underground oceans, hundreds of millions of miles from Earth.

"One of the biggest discoveries in my field is that the solar system is littered with oceans," says Alan Stern, planetary scientist and principal investigator of the New Horizons mission. "The Earth is an oddball; it wears its oceans on the outside. The others have their oceans on the inside, below an insulating layer of ice."

"One of the biggest discoveries in my field is that the solar system is littered with oceans."

It is in these oceans that NASA's ASTEP (Astrobiology Science and Technology for Exploring Planets) program hopes to discover extraterrestrial life. Jupiter's moon Europa is the primary target, although other celestial bodies in our solar system, such as Jupiter's moon Ganymede and Saturn's moons Enceladus and Titan, could also harbor life.

Stone has been working on NASA-funded autonomous underwater vehicles (AUVs) for over a decade, beginning with the DEPTHX (DEep Phreatic THermal eXplorer) research mission to explore the hydrothermal sinkhole Zacatón in the Gulf of Mexico. The microbe detection and collection systems on DEPTHX successfully discovered four new phyla of bacteria, laying the foundation for a future AUV mission to Europa.

In the past months, Stone led a team on the Matanuska glacier in Alaska to test the "cryobot" VALKYRIE (Very deep Autonomous Laser-powered Kilowatt-class Yo-yoing Robotic Ice Explorer). Think of the cryobot as a very advanced cargo van— it's meant to tunnel through surface ice and deliver an AUV to subterranean water systems below.

"The fastest way to get through ice is not to heat a hot plate," says Stone, referring to the idea of heating up a metal sheet under the cryobot and letting it sink. "It is to melt water ahead of the vehicle, pull that melted water up through a heat exchanger, and then run it through high-pressure jets that carve the ice below the vehicle."

"As of 4 or 5 years ago, this would have been science fiction. It's real now, and it works."

This type of cryobot technology is new in the scientific community. "As of 4 or 5 years ago, this would have been science fiction," says Stone. "It's real now, and it works."

Next on the agenda: head to Antarctica to test ARTEMIS (Autonomous Rover/airborne-radar Transects of the Environment beneath the McMurdo Ice Shelf), the fourth AUV constructed by Stone Aerospace. A prep team will reach Antarctica on August 22 to set up a lab on the ice, while the main team will arrive at the end of September with ARTEMIS.

"A lot of things have happened in the last five years to make a subsurface ocean mission to Europa far more feasible than anybody at NASA would have believed just a few years ago," says Stone.

But there are still many pieces to put together. A successful exploration mission of Europa's subsurface oceans would require three main components: a cryobot transport vehicle to tunnel through the surface ice (akin to VALKYRIE), an AUV capable of independently exploring a vast underground water system (similar to ARTEMIS), and multiple "marsupial robots," which are small, disposable AUVs designed to collect samples from hazardous areas and bring them back to the main AUV.

Stone Aerospace testing the VALKYRIE cryobot in the Matanuska glacier in Alaska.

"ARTEMIS is actually kind of the precursor for what we call a planet-class AUV—something that is capable of persistent exploration of the entire subsurface ocean," says Stone. "And that's got to be a fast, long-range vehicle that stays away from problematic areas."

Problematic areas could include "black smoker" vents—hydrothermal vents that emit particles of iron sulfide—or layers of water with drastically different chemical makeups known as "chemoclines." Once the main AUV discovers one of these "high-energy zones," it will release a marsupial bot to collect samples.

"The smaller AUVs spin off when you find something dangerous and interesting, like a volcanic thermal vent or chemically rich energy source. The small vehicle goes over, uses its onboard intelligence to determine what criteria we want for it to select a sample, grabs a sample, and then brings it back to the main planet-class vehicle which has more sophisticated systems to detect and classify life."

Stone Aerospace is working to bring all of these mechanisms together in an overarching project called SPINDLE (Sub-glacial Polar Ice Navigation, Descent, and Lake Exploration).

"SPINDLE is an integration of all these ideas into a functional flight vehicle that will be tested in Antarctica and put into one of the deep subglacial lakes there," explains Stone. "My guess is that we will finish that within five or six years."

But even with all aspects of the craft tested and ready to go—the cryobot, the planet-class AUV, and the sample-collecting AUVs—there is still the challenge of powering all that equipment. NASA generally uses solar panels to power spacecraft, but Europa is too far away from the sun, and this rig will require too much energy for solar power to be feasible.

A radioisotope thermoelectric generator (RTG) can be used when solar energy will not suffice. RTGs are typically filled with plutonium-238 for its high alpha radiation, which produces heat that can be converted into electrical energy, and low beta and gamma radiation, which can be more penetrating and dangerous.

The problem is, NASA plans to use virtually all of its available plutonium-238 for the 2020 rover to Mars, a duplicate of the Curiosity rover.

"There are efforts afoot right now to restart NASA's plutonium-238 production," says Stone. "And there are people who are privately looking into doing that for a commercial mission as well."

Even with a large RTG to power the lander, the cryobot will require an even greater power source, a nuclear reactor, to tunnel through 10 to 20 kilometers of ice.

"The cryobot will need anywhere from 250 to as much as 500 kilowatts of thermal power," explains Stone. "To do that, you'd need a ceramic and metallic encased custom-designed fission reactor. This is not like a power reactor here on Earth. It's a cylinder, roughly half a meter in diameter and a meter long. It is a ceramic encapsulated, zirconium metallic shell with shielding. It has control systems in it that will allow it to heat up and generate roughly half a megawatt worth of thermal power."

Running a nuclear reactor of that size for just a few hours would produce enough energy to power a single-family home for a month. Although a portable fission reactor this powerful has not been constructed, Stone Aerospace is working with the Center for Space Nuclear Research (CSNR) on design concepts.

Before an AUV can be sent to explore extraterrestrial subsurface oceans, NASA's planned Europa Mission will enter orbit around Jupiter and perform several close flybys of the Jovian moon. The goal of the mission is to further study Europa's ocean system and determine whether the moon could harbor life. Scheduled for launch in 2025, the Europa Mission could also scout out a landing site for a future penetrator probe, measure the amount of ice a future cryobot will need to tunnel through, and thereby determine how much power will be required.

"NASA's procedure is to do a flyby mission, then an orbiter, then a lander, then a rover," says Stone. "So if you work that whole scenario out, you might see a Europa penetrator/lander/AUV mission somewhere out around 2050, maybe 2060."

But Stone called the 2025 launch date for the Europa flyby mission, "conservative," and he is more optimistic about a timeframe for exploring Europa's subterranean oceans.

"All of us here have been working steadily for 12 years to make this mission a possibility in our lifetimes," says Stone. "Once we have a successful mission with SPINDLE in Antarctica, there is nothing—I'll repeat, nothing—in the way of a full-blown life search and subsurface ocean investigation on Europa, Enceladus, or Ganymede. It's not science fiction anymore."

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